You may not have heard of hybrid perovskites, but they could make solar panels much cheaper. They could also make X-ray detectors more efficient, which would mean less radiation in scans. Professor of Photophysics and Optoelectronics Maria Antonietta Loi studies these exciting materials and has just published two papers on them.
Perovskite is a mineral that was first discovered in the Urals in 1839 by German mineralogist Gustave Rose. It has a peculiar crystal structure, an octahedron and a cube surrounding a central atom. Many materials proved able to produce the perovskite crystal structure, and they now form a class of their own, the perovskites.
Scientists first discovered that perovskites could be used to make solar cells in 2009. ‘They absorb a lot of light’, says Maria Antonietta Loi. ‘And they exhibit good electronic transport.’ Furthermore, it is easy to grow perovskite crystals and thin films from a solution and the material is very cheap. The first perovskite solar cells were not very efficient, but in just a few years this has been cranked up to 22 percent, which is comparable to the efficiency of high-quality silicon solar cells. ‘The power conversion efficiency has increased very rapidly indeed.’
Maria Antonietta Loi has been studying perovskites for a couple of years now. In a paper published on 8 April in the journal Light Science & Application, Loi, her postdoc Hong-Hua Fang and several colleagues describe perovskite cells made from three different precursors. They looked at the optical properties of the resulting thin films to find out how the nature of the precursors affects not only the solar cells but also the way the thin films interact with light.
‘It is quite a technical paper’, says Loi. ‘I really want to understand their physical properties and use this knowledge to improve the materials.’ The perovskites need to be made more stable, for example. ‘And we want to tune the chemical structure to further increase the efficiency. Many different types of hybrid perovskite can be synthetized, and my colleague Thom Palstra is trying to discover new systems’.
The perovskites Loi used also contain organic molecules, which is why they are called hybrids. The study of organic solar cells has a long tradition at the University of Groningen. ‘We are really building on that experience’, says Loi. But, like all hybrid perovskite solar cells to date, the mixture also contains lead, which is toxic. ‘In the long run this could be a barrier to their widespread use in solar panels, even if lead is allowed for solar application’, says Loi. Nevertheless, adds Fang, ‘Some start-up companies are already fabricating perovskite solar panel prototypes.’
Hybrid perovskites are exceptional, and apart from solar cells, they can also be used to make X-ray detectors. Loi and Fang were part of an international team that described an efficient X-ray detector made from hybrid perovskite single crystals. ‘It is four times more efficient than state-of-the-art detectors’, says Fang.
The energy of the X-rays is turned into a current. Loi: ‘Perovskites contain a lot of heavy atoms, which allow them to absorb the radiation very efficiently.’ Better detectors mean that a lower dose of radiation can be used, in CT scanners for example.
Although making a device with a clear application is fun, for Loi the work is about understanding how perovskites work and what makes them so special. ‘That is the question that connects both papers.’ The paper on the X-ray detectors was published in Nature Photonics on 21 March.
Loi’s recent work on perovskites has made her preeminent in this field. Her papers on the topic have featured several times on the cover of journals, and she wrote a News & Views comment in Nature Materials three years ago, together with her University of Groningen colleague Kees Hummelen, a pioneer in organic solar cells. There is more in the pipeline, both fundamental work and new applications. The synergy inside the Zernike Institute for Advanced Materials is ideal for this. Perovskite sounds like great stuff. ‘It is cheap, you can use it in detectors, solar panels or light emitters, and the method for making crystals and thin films is so basic that you could do it in your kitchen.’
Photoexcitation dynamics in solution-processed formamidinium lead iodide perovskite thin films for solar cell applications. Hong-Hua Fang, Feng Wang, Sampson Adjokatse, Ni Zhao, Jacky Even and Maria Antonietta Loi doi:10.1038/lsa.2016.56
Sensitive X-ray detectors made of methylammonium lead tribromide perovskite single crystals. Haotong Wei, Yanjun Fang, Padhraic Mulligan, William Chuirazzi, Hong-Hua Fang, Congcong Wang, Benjamin R. Ecker, Yongli Gao, Maria Antonietta Loi, Lei Cao & Jinsong Huang, Nature Photonics 21 March 2016 doi:10.1038/nphoton.2016.41
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